1. Field of the Invention
The present invention relates to a two-wheel/four-wheel drive switching device for a vehicle.
2. Description of Background Art
A vehicle capable of switching between a two-wheel drive mode and a four-wheel drive mode is known in the art.
FIGS. 4 and 5 show an example of such a vehicle. In FIGS. 4 and 5, vehicle 1 is composed generally of a body frame 3, an engine 2 mounted on the body frame 3 at its longitudinally central position, a pair of front wheels 4 provided at a front portion of the body frame 3, and a pair of rear wheels 5 provided at a rear portion of the body frame 3. Further, a steering handle 6 is provided at a front upper portion of the body frame 3 for steering the front wheels 4, a fuel tank 7 mounted on the body frame 3 at a position above the engine 2, and a seat 8 is mounted on the body frame 3 at a position behind the fuel tank 7.
As shown in FIG. 5, the front wheels 4 are vertically movably supported by a pair of front suspensions 9 mounted on the body frame 3 at its front side portions, and the rear wheels 5 are also vertically movably supported by a pair of rear suspensions 10 mounted on the body frame 3 at its rear side portions.
A final reduction gear unit 13 for the front wheels 4 is provided at a front central portion of the body frame 3 and connected through a propeller shaft 11 to the engine 2. In a similar manner, a final reduction gear unit 14 for the rear wheels 5 is provided at a rear central portion of the body frame 3 and connected through a propeller shaft 12 to the engine 2. The front wheels 4 are connected to the final reduction gear unit 13, and the rear wheels 5 are connected to the final reduction gear unit 14.
As shown in FIG. 5, a two-wheel/four-wheel drive switching device 15 for switching between the continuation and interruption of power transmission to the front wheels 4 to thereby switch between a rear-wheel drive mode and a four-wheel drive mode is provided between the final reduction gear unit 13 and the propeller shaft 11.
Alternatively, the two-wheel/four-wheel drive switching device 15 may be provided between the propeller shaft 12 and the final reduction gear unit 14 to switch between a front-wheel drive mode and a four-wheel drive mode.
Such two-wheel/four-wheel drive mode switching is carried out based on a road condition or a running condition, for example.
FIG. 9 shows an example of a conventional two-wheel/four-wheel drive switching device 15.
The two-wheel/four-wheel drive switching device 15 shown in FIG. 9 has an input shaft 16 connected to the final reduction gear unit 13. The input shaft 16 is axially divided into two input shaft components 16a and 16b aligned and butted together. The device 15 further includes a switching unit 17 for switching between the connection and disconnection of the input shaft components 16a and 16b. 
In more detail, a cylindrical positioning projection 18 is formed centrally on an end surface of the input shaft component 16a located on the final reduction gear unit 13 side, and a positioning recess 19 is formed centrally on an end surface of the input shaft component 16b located on the engine 2 side. The positioning projection 18 of the input shaft component 16a is rotatably engaged with the positioning recess 19 of the input shaft component 16b. Accordingly, the two input shaft components 16a and 16b are butted together so that the positioning projection 18 and the positioning recess 19 are engaged with each other, thereby making an alignment of the input shaft components 16a and 16b and a relatively rotatable connection therebetween.
Further, a spline S1 is formed on the outer circumferential surface of the input shaft component 16a near its butted end surface, and a spline S2 is formed on the outer circumferential surface of the input shaft component 16b near its butted end surface. The switching unit 17 is provided so as to surround the splines S1 and S2 of the input shaft components 16a and 16b. 
The switching unit 17 is composed of a switching ring 20 having an internal spline slidably meshing with the splines S1 and S2, and a driving mechanism 21 configured by a solenoid for sliding the switching ring 20 in the axial direction of the input shaft components 16a and 16b to select a first position where the internal spline of the switching ring 20 meshes with only the spline S1 of the input shaft component 16a or a second position where the internal spline of the switching unit 20 meshes with both the splines S1 and S2 of the input shaft components 16a and 16b. 
In operation, when the switching ring 20 is axially moved in one direction by the driving mechanism 21 to engage with only the input shaft component 16a, the transmission of power from the engine 2 to the front wheels 4 is interrupted to thereby obtain a two-wheel drive mode where only the rear wheels 5 are driven. On the other hand, when the switching ring 20 is axially moved in the other direction from the above condition by the driving mechanism 21 to engage with both the input shaft components 16a and 16b, the input shaft components 16a and 16b are connected together to transmit the engine power also to the front wheels 4, thereby obtaining a four-wheel drive mode where both the front wheels 4 and the rear wheels 5 are simultaneously driven.
As described in Japanese Patent Laid-open No. Hei 8-216714, the two-wheel/four-wheel drive switching device 15 is maintained in an energized condition in the four-wheel drive mode.
As shown in FIG. 10, the supply of a drive current to the two-wheel/four-wheel drive switching device 15 is kept stopped in the two-wheel drive mode, and the drive current is continuously supplied to the device 15 in the four-wheel drive mode.
In the conventional two-wheel/four-wheel drive switching device 15 mentioned above, the following problems to be solved remain.
If there is any difference in peripheral speed between the front wheels 4 and the rear wheels 5 in switching the drive mode, the spline of the switching ring 20 does not mesh effectively with the spline S2 of the input shaft component 16b, causing a problem that the two-wheel drive mode cannot be switched to the four-wheel drive mode.
Even though the drive mode switching is effected, the splines of the switching ring 20 and the input shaft component 16b strike each other to cause the generation of noise.
To solve these problems, it is necessary to provide a mechanism for synchronizing (aligning) the splines of the switching ring 20 and the input shaft component 16b, causing complication of the structure. Further, in mounting the two-wheel/four-wheel drive switching device 15, the structure of any member at a mounting position must be substantially modified from the existing structure.
Further, in the technique described in Japanese Patent Laid-open No. Hei 8-216714 mentioned above, a drive current is continuously supplied to the two-wheel/four-wheel drive switching device 15 in the four-wheel drive mode, causing an increase in power consumption, which becomes a heavy burden for a small-sized vehicle.